1. Field of the Invention
The present invention relates to a microcomputer in which an input unit, an output unit, a central processing unit and a memory unit are incorporated in a single chip, i.e. one-chip microcomputer and more particularly pertains to a general-purpose one-chip microcomputer which can be adapted for various requirements of the users who purchase the microcomputer, incorporate it in their product and manufacture and sell the product.
2. Description of the Prior Art
The input/output (I/O) terminals of conventional general-purpose one-chip microcomputers can be optionally specified as to the type of output terminal and can be determined to one of (1) N-channel, (2) P-channel and (3) C-MOS at the masking stage in the process of producing the microcomputer. However, in such conventional microcomputers, only a few terminals are provided as interrupt terminals. Hence, in the case when more than a few interrupt terminals were required, gate circuits were provided outside of the microcomputer, or a custom-made microcomputer designed for the particular purpose to have a desired number of interrupt terminals had to be employed.
When a custom-made microcomputer is employed, its unit price is high and this increases the price of the product including the microcomputer since the manufacture of the microcomputer can not expect mass production effect from the custom-made microcomputer. When the gate circuit is provided outside of the microcomputer, the cost of the outer circuit will also increase.
Additionally, there has been no general-purpose microcomputer in which, besides the number of interrupt terminals, the polarity of interrupt at each interrupt terminal can be specified or in which the interrupt terminal can be masked.